1. Field of the Invention
The present invention relates to an apparatus for retensing a pre-stressed structure, and in particular to an apparatus for retensing a pre-stressed structure which is capable of recovering a deformed prestressed structure to its original state by retensing tensing members disposed in the interior of a prestressed structure having a predetermined stress loss.
2. Description of the Conventional Art
A prestress force in a prestressed structure such as a prestressed concrete beam, etc. is gradually decreased by a predetermined loss factor such as a creep of a concrete, a dehydration-based contraction, a relaxation of a prestressed concrete member, etc.
In the case that the loss of a stress is gradually increased in a structure such as a bridge beam, the bridge may be slacked. The slacking of the bridge beam may cause a critical damage to the beam as well as the bridge structure. In this case, a proper maintenance or repair is urgently needed for preventing a further accident such as a collapse of the bridge.
In the conventional prestressed structure such as a prestressed concrete beam, a plurality of strand cones are installed at both ends of a beam structure in which both ends of the strand are exposed when manufacturing the prestressed concrete beam, and the tensing of the strand is implemented using a tension apparatus. Thereafter, the tension apparatus is removed, and a cement mortar is injected into a sheath tube having a strand therein.
FIGS. 9 and 10 illustrate a bridge structure in which a conventional prestressed concrete beam is installed. In a bridge prestressed concrete beam 1, longitudinal ends of each beam are installed on the upper portion of a supporting apparatus 3 installed on the upper portions of the piers 2. An upper plate 4 is mounted on the upper surface of each prestressed concrete beam 1.
In the thusly constituted conventional prestressed concrete beam 1, a cement mortar is filled at the end portion of each sheath tube 6 into which a strand formed of a plurality of wire strands is inserted, and a plurality of fixing members 7 are exposed into concave space portions 1a formed at the end portions of the beam 1. In the above-described structure, it is impossible to retense each strand fixed to the fixing member 7 through the space portion 1a formed between the end portions of each beam structure. In addition, since a cement mortar is filled into the space portion 1a through which each fixing member 7 is exposed, it is impossible to correct the deformation of the beam structure due to the retensing of the inner strand disposed in the interior of the beam structure when a slack occurs at the beam 1.
Therefore, as a technique for repairing the bridge beam in which a slack occurs, the following technique is well known.
A plurality of through holes are horizontally formed through the beam structure in the direction of its width at both ends of the prestressed concrete beam in which the slacking occurs, and a reinforcing steel member is inserted and buried into each through hole. Thereafter, another external reinforcing steel member is installed in the vertical direction at the ends of each buried steel member extended from both walls of the beam structure, and then a cement concrete is formed at the engaged portions of the external reinforcing steel member for thereby forming a concrete support portion extended from both external sides of the beam structure.
Next, a fixing member supporting the end portions of the strands is installed at an end portion of the concrete support member, and the end portion of each strand is fixed at the fixing member along both side lateral walls of the beam structure, and the end portions of each strand formed of a plurality of wire strands are tensed using a tension apparatus, and the deformed portion of the beam structure is recovered by lifting up the deformed portion of the same for thereby correcting the slacking of the bridge beam and the upper plate, so that the strength of the beam structure is enhanced.
However, in this conventional bridge beam maintenance technique, when installing a plurality of reinforcing strands along both side lateral surfaces of the beam structure, and installing the support member such as a protrusion support member, an external fixing member, etc., the construction period is long, and the fabrication process is complicated. In addition, since a plurality of through holes are formed in the beam structure, the strength of the beam is decreased, so that it is impossible to implement a stable bridge structure.
In addition, in the conventional fixing member in which a plurality of wire strands are supported, if a relaxation or a predetermined damage occurs in the steel member, it is impossible to individually retense the deformed member or to exchange the deformed member with a new one. If one strand is cut, the entire strands may be cut for thereby causing a large accident.
In order to overcome the above-described problems encountered in the conventional bridge maintaining and reinforcing technique, the inventor of this invention discloses a patent application entitled "Method and apparatus for maintaining a bridge beam" in 1994 having its Korean Patent Laid-open publication No. 1996-4668. In this application, both end support brackets in which a plurality of fixing members are installed are fixed at both ends of the prestressed concrete beam for correcting a deformation. Both ends of each strand supported by the lateral surface support bracket installed at an intermediate portion of the beam structure are tensed and fixed by each collet provided at the support member of the fixing member, for thereby effectively maintaining the prestressed concrete beam of an aged bridge.
According to the above-described method and apparatus for maintaining a bridge beam, it is possible to prevent the decrease of the strength of the beam structure which may be caused due to a plurality of through holes formed at both end portions of the prestressed concrete beam. In addition, the strength of the support portion of the fixing member supporting both ends of each reinforcing strand is significantly increased. In addition, the maintaining period may be decreased compared to the conventional technique. Since it is possible to individually repair a deformed or slacked strand supported by the external fixing member and newly add a predetermined number of strands thereto, even when a large stress occurs in the strand, the entire strands are not affected for thereby implementing an easier maintenance and exchange of the deformed or slacked beam.
However, in the bridge beam maintaining method based on the conventional maintaining method and above-described Korean patent application, the construction is relatively expensive, and it is difficult to check whether or not the retensing is needed for the structure when the tensed strand is relaxed, namely, to perform the stability test when a predetermined stress loss occurs in the strand. In more detail, when adapting the prestress structure to the bridge beam, in order to check whether or not the retensing is needed for the strand when the beam is deformed, a predetermined measuring method such as a vehicle loading test is performed. In this case, the loading test is irregularly performed for thereby retensing the deformed structure. If the above-described loading test is missed, the strength of the beam structure may be significantly decreased, so that the recovering of the beam structure may not be implemented by the repair or maintenance. As a result, in order to prevent the breakage or collapse of the bridge, the entire bridge structure may be rebuilt.
In this case, the bridge may not be used for a predetermined period for the vehicle loading test resulting in a large traffic jam in other areas.